This study addresses the question, "Does diet alter the composition and concentration of serum lipoproteins?" Since strict adherence to diet for extended periods of time are planned, the baboon, an excellent animal model, will be used in this study in place of man. A carefully designed sequence of diets will be fed one after the other for periods of three months each. The diets will contain the following components, known to alter serum lipid concentrations, either alone or in combination: fats (coconut oil, lard and corn oil), cholesterol and sucrose. The fats were chosen to provide a range saturation from coconut oil, the most saturated, to corn oil, the most unsaturated. During each dietary period, the animals will be bled a sufficient number of times in order to follow the rate of change in the composition and concentration of the lipoproteins. The lipoproteins are separated from serum by ultracentrifugal techniques and analyzed for the following parameters: lipid (unesterified and esterified cholesterol, phospholipids; triglycerides), protein composition, apoprotein distribution, distribution of each phospholipid class and the fatty acid analysis of each lipid and phospholipid class. The results of this study to date indicate the following: fat containing diets give higher serum cholesterol values than low fat diets, the more unsaturated the fat in the diet the lower the resulting serum cholesterol values and the more saturated the fat in the diet the higher the High Density Lipoprotein cholesterol concentrations. This and future data will clearly show the influence of each diet, which in turn will provide a strong basis for recommendations of diets which may alter serum lipoprotein concentrations in man.